4.5.1Additional lessons
Osmosis — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 113 of 142
Part 1 of 5Water as a Solvent
Osmosis only makes sense once you know why water dissolves so many substances.
Dipolar molecule
- Oxygen pulls shared electrons closer than hydrogen does,
- so each water molecule has a slightly negative oxygen end and slightly positive hydrogen ends.
Solvation
- Water molecules cluster around charged or polar solute particles, attracted by hydrogen bonds or ion–dipole forces, and pull them apart.
Hydration shell
- The layer of oriented water molecules surrounding each dissolved particle,
- which keeps the solute dispersed and stops it re-forming a solid.
Solution
- A solute dissolved in a solvent;
- in living cells the solvent is nearly always water and the solutes include ions, sugars and amino acids.
| Particle type | Interaction with water |
|---|---|
| Ions (Na⁺, Cl⁻) | Attracted to oppositely charged ends of water; surrounded by hydration shells |
| Polar molecules (glucose) | Hydrogen bonds form with the hydroxyl groups |
| Non-polar molecules (lipids) | No strong attraction, so they do not dissolve |
Part 2 of 5What Osmosis Is
What Osmosis Is
Definition
- Osmosis is the net movement of water molecules across a partially permeable membrane from a region of lower solute concentration to one of higher solute concentration.
Membrane role
- The membrane lets small water molecules pass but blocks larger solute molecules, so only water can equalise the concentrations.
Passive process
- No ATP is needed because water moves down its own gradient using the kinetic energy of its molecules.
Water potential
- Water potential, Ψ, measures the tendency of water to move;
- water moves from higher Ψ to lower Ψ, and added solute lowers Ψ.
Two-way traffic
- Water molecules cross in both directions all the time;
- osmosis describes the overall difference between the two directions.
Part 3 of 5Comparing Solutions
Tonicity always compares one solution with another, and the outside solution is named relative to the cell.
Hypotonic
- The surrounding solution has a lower solute concentration than the cell, so water enters the cell by net movement.
Hypertonic
- The surrounding solution has a higher solute concentration than the cell, so water leaves the cell by net movement.
Isotonic
- Both solutions have equal solute concentration, so water moves in and out at equal rates and there is no net movement.
| Outside solution | Solute concentration outside | Net water movement |
|---|---|---|
| Hypotonic | Lower than cell | Into the cell |
| Isotonic | Equal to cell | None |
| Hypertonic | Higher than cell | Out of the cell |
Part 4 of 5Predicting the Direction
1
Identify the two solutions separated by the membrane and decide which has the higher solute concentration.
2
Remember that the solution with more solute has the lower water potential, because its water is less free to move.
3
State that water moves by net osmosis from the dilute solution towards the more concentrated solution.
4
Check the membrane is partially permeable and the solute cannot cross; if it could, the solute would diffuse as well.
5
Describe the result using comparative language such as net movement, down the water potential gradient, and equilibrium.
Part 5 of 5Exam Checklist & Tips
4.5.1 – Key Points for the Exam
- Osmosis Net movement of water across a partially permeable membrane from lower to higher solute concentration.
- Partially permeable membrane A barrier that allows some substances, such as water, through but restricts others.
- Hypotonic A solution with lower solute concentration than the cell or solution it is compared with.
- Hypertonic A solution with higher solute concentration than the one it is compared with.
- Isotonic Solutions with equal solute concentration, giving no net water movement.
- Water potential The tendency of water to move from one place to another, highest in pure water.
Common Mistakes
- Saying that water stops moving in an isotonic solution;
- molecules keep crossing, but there is no net movement.
- Stating that water moves towards where there is more water; it moves towards the higher solute concentration.
- Describing the membrane as impermeable or semi-permeable without saying partially permeable, which is the term used in the course.
- Mixing up hypotonic and hypertonic; always ask which solution is more dilute compared with the cell.
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